Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1359063

Re: [PATCH] Revert "arm64: Increase the max granular size"

Show all headers | View raw

On Wed, Mar 16, 2016 at 02:35:35PM +0000, Will Deacon wrote:
> On Wed, Mar 16, 2016 at 02:03:35PM +0000, Mark Rutland wrote:
> > If I understand correctly, the main reason that we need this for correctness is
> > non-coherent DMA to/from SLAB caches.
> > 
> > A more general approach (and more invasive, but perhaps less so than making
> > ARCH_DMA_MINALIGN usage completely dynamic) would be to determine at runtime
> > whether the CWG is larger than the configured ARCH_DMA_MINALIGN, and if so,
> > force the use of bounce buffers (which could be padded to the architectural
> > maximum of 2K) for non-coherent DMA. That nicely degrades to not mattering for
> > the case of coherent DMA.
> >
> > I would consider NoSnoop a separate case. It's closer to "negatively coherent",
> > and always required page-aligned buffer anyway due to MMU behaviour.
> 
> What makes you say that? There are no such alignment requirements for
> buffers that may be accessed with a NoSnoop transaction. On ARM, we'll
> have a mismatched alias, but we'd need to solve that with explicit
> cache maintenance (and my understanding is that's what things like GPU
> drivers already do on x86).

I was under the impression that NoSnoop transactions were permitted to be
Cacheable, even if non-snooping (e.g. allowing them to allocate and hit in a
system cache).

If that is permitted, then data corruption could potentially occur in the
presence of another cacheable alias due to things like line migration (e.g. a
CPU making a speculative fetch and taking ownership of a line that was in the
system cache). To avoid that, you'd have to remove any cachable alias, for
which we only have page-granular control.

If that is not permitted, then no-snoop is effectively non-cacheable and
non-coherent, and my comment doesn't hold.

Thanks,
Mark.

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH] Revert "arm64: Increase the max granular size" Ganesh Mahendran <opensource.ganesh@gmail.com> - 2016-03-16 11:00 +0100
  Re: [PATCH] Revert "arm64: Increase the max granular size" Will Deacon <will.deacon@arm.com> - 2016-03-16 11:10 +0100
    Re: [PATCH] Revert "arm64: Increase the max granular size" Timur Tabi <timur@codeaurora.org> - 2016-03-16 14:10 +0100
      Re: [PATCH] Revert "arm64: Increase the max granular size" Mark Rutland <mark.rutland@arm.com> - 2016-03-16 15:10 +0100
        Re: [PATCH] Revert "arm64: Increase the max granular size" Will Deacon <will.deacon@arm.com> - 2016-03-16 15:40 +0100
          Re: [PATCH] Revert "arm64: Increase the max granular size" Mark Rutland <mark.rutland@arm.com> - 2016-03-16 16:00 +0100
      Re: [PATCH] Revert "arm64: Increase the max granular size" Catalin Marinas <catalin.marinas@arm.com> - 2016-03-16 15:20 +0100
        Re: [PATCH] Revert "arm64: Increase the max granular size" Timur Tabi <timur@codeaurora.org> - 2016-03-16 16:30 +0100
          Re: [PATCH] Revert "arm64: Increase the max granular size" Catalin Marinas <catalin.marinas@arm.com> - 2016-03-17 15:30 +0100
            Re: [PATCH] Revert "arm64: Increase the max granular size" Timur Tabi <timur@codeaurora.org> - 2016-03-17 16:00 +0100
              Re: [PATCH] Revert "arm64: Increase the max granular size" Catalin Marinas <catalin.marinas@arm.com> - 2016-03-17 16:40 +0100
                Re: [PATCH] Revert "arm64: Increase the max granular size" Marc Zyngier <marc.zyngier@arm.com> - 2016-03-17 17:10 +0100

csiph-web